This invention relates to a process for the production of high-strength and thermal-shock-resistant ceramic of phosphate, [ZrO].sub.2.P.sub.2 O.sub.7 or 2ZrO.sub.2.P.sub.2 O.sub.5.
In the ZrO.sub.2 --P.sub.2 O.sub.5 -system many compounds are known, i.e. zirconium phosphate (ZrO.sub.2).P.sub.2 O.sub.7 or 2ZrO.sub.2.P.sub.2 O.sub.5), NaZr.sub.2 P.sub.3 O.sub.12 --type compounds, hydrated-compounds of this system and amorphous materials of this system. They have different chemical composition and different crystal structure. They are classified into two groups. The first two are used as low-thermal-expension ceramics, though the refractory temperature of (ZrO).sub.2.P.sub.2 O.sub.7 is higher (1700.degree. C.) than that of NaZr.sub.2 P.sub.3 O.sub.12 (1300.degree. C.). The last two are used as catalyzer or special-function-ceramics.
It has hitherto been known that a ceramic of phosphate polycrystalline ceramic was low-thermal-expansive. However, its powder compact alone has not densified and it has not been known whether the low thermal expansion of the sintered ceramic was due to the crystal itself or due to microcracks in the ceramic like aluminum titanate.
The inventor has found that the powder compact of zirconyl phosphate was able to be densified when a metal oxide was used as densifying aid, the crystal has a low thermal-expansion-coefficient, 1.7.times.10.sup.-6 /.degree.C., and the transition grain size for microcracking was 7 .mu.m (J. Amer. Ceram. Soc., Vol. 68, No. 5, May 1985, pp. 273-278).
Moreover, the inventor has found that a high-strength and thermal-resistant ceramic of zirconyl phosphate could be produced when he used a grain-growth-suppressing aid, and that the solid solution of zirconyl phosphate with niobium oxide had a lower thermal-expansion-coefficient, 0.3.times.10.sup.-6 /.degree.C. at 5 wt% niobium oxide, than the pure phase. The present invention is based on these findings.